The cost to buy a domain varies based on the extension and whether it's registered already or not. If you are purchasing a new domain that's not been registered the price can vary from as low $12 to $100 retail price.
If you are puchasing the a domain that's already registered the price can vary from as low as $12 to millions depending on how much the seller is willing to sell it for. This option will probably needs it's own article to explain the process on how domains are appraised.
To check the cost of both new and registered domains just start a search on our home for the name you are looking for.